Semiconductor devices often require thin oxide layers to be formed at various stages of their fabrication. For example, in transistors, a thin gate oxide layer may be formed as part of a gate stack structure. In addition, in some applications, such as in the fabrication of a flash memory film stack, a thin oxide layer may be formed surrounding the entire gate stack (referred to herein as pure oxidation), for example, via exposing the stack to an oxidation process. Such oxidation processes have conventionally been performed either thermally or using a plasma. In other applications, the oxide layer may be selectively formed only on certain layers of a film stack (referred to herein as selective oxidation).
Conventional thermal processes for forming oxide layers, for example, a gate oxide layer or a gate stack oxide layer, have worked relatively well in fabrication of semiconductor devices of the larger feature sizes used in the past. Unfortunately, as feature sizes are becoming much smaller and different oxides are employed in the next generation of advanced technologies, the high wafer temperatures required in thermal oxidation processes are problematic in that the sharp junction definitions which are now required become diffused at the higher temperatures (e.g., above about 700 degrees Celsius). Such a distortion of junction definitions and other features can lead to poor device performance or failure.
Plasma processes used to form oxide layers have similar problems. For example, at high chamber pressure (e.g., 100 mTorr), contaminants tend to accumulate in the gate oxide layer during formation, leading to fatal defects in the gate oxide structure such as dangling bonds or mobile charge, and at low chamber pressure (e.g., tens of mTorr), increased plasma ion energy leads to ion bombardment damage and other diffusion problems.
For example, conventional oxidation processes often result in a defect known as a bird's beak. Bird's beak refers to diffusion of the oxide layer into the layers of the film stack structure from the sides at the interface between adjacent layers, rounding off the corners of the adjacent layers. The resultant defect has a profile that resembles a bird's beak. The intrusion of the oxide layer into the active region of the memory cell (e.g., in flash memory applications) reduces the active width of the memory cell, thereby undesirably reducing the effective width of the cell and degrading the performance of the flash memory device.
In addition, in some film stack structures comprising both metal and non-metal containing layers, such as DRAM memory devices, sidewall oxidation of the non-metal containing layers may be desired as, in certain application, oxidation of the metal containing layers may limit electrical conductivity and reduce device function. For example, conventionally, selective sidewall oxidation in the presence of tungsten (W) metal may be achieved by using a mixture of hydrogen (H2) and oxygen (O2) gases which react in-situ at high pressures to produce oxidizing species or by using a mixture of water vapor (H2O) and hydrogen (H2). However, these conventional methods are not sufficient to achieve selective sidewall oxidation conditions in the presence of certain metal alloys, such as titanium nitride (TiN), as the process will undesirably oxidize the TiN.
Thus, there is a need for improved methods for oxidizing stacks of materials.